[maverick, maverick/ti-omap4, natty, natty/ti-omap4, oneiric, precise CVE 1/1] mm: memcg: Correct unregistring of events attached to the same eventfd

Andy Whitcroft apw at canonical.com
Mon Mar 12 11:22:11 UTC 2012

From: Anton Vorontsov <anton.vorontsov at linaro.org>

There is an issue when memcg unregisters events that were attached to
the same eventfd:

- On the first call mem_cgroup_usage_unregister_event() removes all
  events attached to a given eventfd, and if there were no events left,
  thresholds->primary would become NULL;

- Since there were several events registered, cgroups core will call
  mem_cgroup_usage_unregister_event() again, but now kernel will oops,
  as the function doesn't expect that threshold->primary may be NULL.

That's a good question whether mem_cgroup_usage_unregister_event()
should actually remove all events in one go, but nowadays it can't
do any better as cftype->unregister_event callback doesn't pass
any private event-associated cookie. So, let's fix the issue by
simply checking for threshold->primary.

FWIW, w/o the patch the following oops may be observed:

 BUG: unable to handle kernel NULL pointer dereference at 0000000000000004
 IP: [<ffffffff810be32c>] mem_cgroup_usage_unregister_event+0x9c/0x1f0
 Pid: 574, comm: kworker/0:2 Not tainted 3.3.0-rc4+ #9 Bochs Bochs
 RIP: 0010:[<ffffffff810be32c>]  [<ffffffff810be32c>] mem_cgroup_usage_unregister_event+0x9c/0x1f0
 RSP: 0018:ffff88001d0b9d60  EFLAGS: 00010246
 Process kworker/0:2 (pid: 574, threadinfo ffff88001d0b8000, task ffff88001de91cc0)
 Call Trace:
  [<ffffffff8107092b>] cgroup_event_remove+0x2b/0x60
  [<ffffffff8103db94>] process_one_work+0x174/0x450
  [<ffffffff8103e413>] worker_thread+0x123/0x2d0

Cc: stable <stable at vger.kernel.org>
Signed-off-by: Anton Vorontsov <anton.vorontsov at linaro.org>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com>
Cc: Kirill A. Shutemov <kirill at shutemov.name>
Cc: Michal Hocko <mhocko at suse.cz>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>

(cherry picked from commit 371528caec553785c37f73fa3926ea0de84f986f)
BugLink: http://bugs.launchpad.net/bugs/952828
Signed-off-by: Andy Whitcroft <apw at canonical.com>
 mm/memcontrol.c |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 20a8193..ebca7c0 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3647,6 +3647,9 @@ static void mem_cgroup_usage_unregister_event(struct cgroup *cgrp,
+	if (!thresholds->primary)
+		goto unlock;
 	usage = mem_cgroup_usage(memcg, type == _MEMSWAP);
 	/* Check if a threshold crossed before removing */
@@ -3695,7 +3698,7 @@ swap_buffers:
 	/* To be sure that nobody uses thresholds */

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